In USCS, what do the letters GM and GC signify in describing soil?

• A. GM indicates silty gravel; GC indicates clayey gravel
• B. GM indicates clayey sand; GC indicates silty sand
• C. GM indicates well-graded; GC indicates poorly graded
• D. GM indicates silica-rich; GC silica-poor

Answer: (A)

In USCS, a two-letter code describes the coarse-grained portion of a soil and any fines present. The first letter names the dominant coarse material (G for gravel, S for sand, etc.), and the second letter names the type of fine material present (M for silt, C for clay). So a code with G as the first letter and M as the second means a gravel containing silt, i.e., silty gravel. A code with G as the first letter and C as the second means a gravel containing clay, i.e., clayey gravel.

That makes sense for the given codes: GM is silty gravel and GC is clayey gravel. The other interpretations would either change the base material (sand rather than gravel), refer to grading rather than fines, or mention properties not used in this notation (like silica content), which aren’t part of the USCS two-letter system.